[Draft]: auto diff net rates

[pmocz]

No description provided.

[Debraheem]

@pmocz this is where we define the rate_screened and its derivates. When auto_diff is implemented into rates, we can just set one value n% rate_screened_ad %val = n% rate_screened, instead of all three calls. Then our derivatives won't be singular values for n% rate_screened_ad %d1val1, rather they will carry the entire interpolating function insiden% rate_screened_ad which can be differentiated at any moment to yield the derivative. This helps later on when these derivatives are used to calculate the derivative of the equilibrium links.